Method and device for the continuous coagulation of aqueous dispersions of graft rubbers

ABSTRACT

A process for the continuous coagulation of aqueous dispersions of graft rubbers suitable for toughening thermoplastics is described. In the process, dispersions are transported through an apparatus having at least one shear element with a slotted stator and a rotating of the rotor within the stator, so that said dispersions are passed radially from the inside to the outside as a result of the rotation of the rotor in the shear element and, during or after passage through the slots of the rotor and stator, are subjected to such strong shearing that they coagulate, resulting in graft rubber coagulums which can be readily worked up even at a solids content of more than 50% by weight of elastomers.

This application is a 371 of PCT/EP97/07261 filed Dec. 23, 1997.

The invention relates to a process for the continuous coagulation ofaqueous graft rubber dispersions in an apparatus having at least onestator/rotor combination as a shear element, and an apparatus for thispurpose.

It is known that the impact strength of hard thermoplastic polymers canbe increased by mixing them with graft rubbers. The resulting polymerblends, for example as ABS or ASA polymers, play an important industrialrole in the production of industrial plastics articles. Such graftrubbers which serve for toughening hard thermoplastics have long beenknown as impact modifiers in the plastics industry. They are graftcopolymers in which a particle shell (P2) of monomers forming hardthermoplastics is grafted onto soft particle cores of rubbers (P1), suchas elastomeric diene polymers or elastomeric acrylate polymers. Thegraft shell (P2) is prepared by polymerization or copolymerization ofmonomers or monomer mixtures forming the hard graft shell (P2) by mass,solution, suspension or emulsion polymerization in the presence of thesoft elastomer or elastomers (P1). It is preferable to carry out thegraft polymerization, at least in the final phase, as a polymerizationin aqueous emulsion, anionic emulsifiers preferably being present. Thepreparation of the graft rubbers thus gives a large amount of aqueousgraft polymer dispersions which can be worked up to give the solid graftpolymers.

As disclosed, for example, in DE-A 3149358 or EP-B 459161, the aqueousgraft rubber dispersions are as a rule precipitated by the addition ofcoagulants, generally used coagulants being aqueous solutions ofwater-soluble inorganic or organic acids and/or salts thereof, such asalkali metal and alkaline earth metal chlorides, sulfates or phosphates,eg. calcium chloride or magnesium sulfate solutions. The coagulation orprecipitation is generally carried out batchwise but may also beeffected continuously (EP-B 459161). The precipitated coagulums whichhave been filtered off are then washed and dried in a known manner. Aknown disadvantage of precipitation by the addition of acids or salts ascoagulants is that impurities often remain in the worked-up polymers andmay impair the product properties.

DE-C 2917321 discloses a process for isolating polymers which have asoftening range above 100° C. from an aqueous emulsion, the aqueousemulsion being coagulated in an extruder by shearing and/or heating totemperatures above the softening range of the polymers, the coagulumthen being fused and being discharged in a hot state under pressure fromthe extruder. The water is then separated off in a subsequent processstep. The process is very energy-consumptive and requires acounter-rotating intermeshing twin-screw extruder for the precipitation.Moreover, ammonium acetate is used as an assistant for accelerating thecoagulation.

U.S. Pat. No. 3,821,348 describes a process in which mixtures of from 70to 80% by weight of acrylonitrile (AN) and from 20 to 30% by weight ofmethyl acrylate (MA) are polymerized in emulsion with subsequentaddition of 12% by weight of butadiene or in the presence of 9% byweight of butadiene/acrylonitrile rubber (based in each case on the sumof the amounts of AN+MA). The resulting acrylonitrile copolymerdispersions or graft polymer dispersions are coagulated to a paste in aWaring mixer as the shearing apparatus and then shaped in an extruder,through a fine die, the thin rods which are introduced into hot water.The product is then washed, dried and processed in a compression mold at150° C. to give rods. The description of the process is limited tocopolymers having a high acrylonitrile content and very low content ofelastomeric butadiene/acrylonitrile rubber.

In U.S. Pat. No. 4,668,738, which refers to the prior art of theabove-mentioned U.S. Pat. No. 3,821,348, it is stated that aqueousdispersions having a rubber content of more than 50% by weight of thesolids content give readily processible products only when dispersionsof styrene/acrylonitrile copolymers as a hard component, in amountswhich reduce the rubber content of the copolymer blend to, preferably,less than 50% by weight (based on the total solids content), are mixedwith the dispersions before their coagulation by shearing. Adisadvantage of the process is that graft rubber dispersions which areprepared for toughening thermoplastics generally have a rubber contentof more than 50% by weight of the solids content and can therefore beprocessed after the process only with the addition of dispersions of thehard component, ie. with a change of the overall composition of thepolymers.

It is an object of the present invention to provide an economicalprocess for the continuous coagulation of aqueous dispersions of graftrubbers which are suitable for toughening hard thermoplastics in thepreparation of ABS and in particular ASA polymers, which process can becarried out without the addition of chemical coagulants and without theaddition of other polymer dispersions, so that the polymers in thecoagulated emulsions have the same overall composition as the polymersin the starting emulsions. Furthermore, in the process dispersions ofgraft rubbers having a rubber content of more than 50% by weight, basedon the solids content, should also be capable of being coagulated togive moist free-flowing powders or moist sludge which can readily beworked up, without them having to be mixed with hard acrylonitrilecopolymer dispersions prior to coagulation.

We have found that these objects are achieved in general if the graftrubber dispersions are transported through an apparatus having at leastone shear element which has a circular slotted stator and a slottedrotor rotating within the stator, and are thereby subjected to theaction of shearing sufficiently strong for coagulation of the dispersionby shearing.

The present invention therefore relates to a process for the continuouscoagulation of aqueous dispersions of graft rubbers (P) which aresuitable for toughening thermoplastics and

a) contain, as soft grafting base (P1), elastomeric polymers and/orcopolymers of 1,3-dienes or acrylic esters having a glass transitiontemperature T_(g) of less than −10° C. in an amount of from 30 to 85% byweight of the amount of graft rubber (P) and

b) contain a hard one-shell or multishell graft shell (P2) comprisingmonomer units which form a thermoplastic polymer or copolymer having aglass transition temperature T_(g) of more than +50° C. and whosecontent of acrylonitrile and/or methacrylonitrile monomer units is from0 to 45% by weight, and

c) at least in the last stage of the graft polymerization, were preparedby emulsion polymerization of the monomers for the graft shell (P2) inthe presence of the grafting base (P1),

wherein the graft rubber dispersion is transported through an apparatushaving at least one shear element with a stationary circular slottedstator and a slotted rotor which rotates within the stator and ismounted on a rotatably mounted drive shaft, so that the graft rubberdispersion introduced into the apparatus is passed radially from theinside to the outside as a result of the rotation of the rotor in theshear element and, during or after passage through the slots of therotor and stator, is subjected to shearing which is so strong thatcoagulation of the graft rubber dispersion takes place. The presentinvention furthermore relates to the use of suitable apparatuses for thecoagulation of graft rubber dispersions by shearing and to an apparatuswhich is modified by displacer elements and is intended for dispersingor coagulating dispersions.

In the novel process, the graft rubber dispersion, which is preferablyintroduced, in particular sucked, axially into the apparatus (10), iscoagulated in shear elements (13, 14, 15) comprising stator/rotorcombinations. Slotted rotors (13 b, 14 b, 15 b) are mounted on arotatably mounted drive shaft (12) within the circular stators (13 a, 14a, 15 a) which are generally firmly connected to the housing (11) of thedispersing apparatus (10), are in the form of disks and preferablyessentially pot-like, and have slots or teeth, the rotors being capableof rotating at high speed within the stator circles. For this purpose,the drive shaft is connected to a motor. The stream of the graft rubberdispersion fed centrally to the first rotor (13 b) is caused to rotateby the rotor (13 a) rotating at high speed and is transported radiallyfrom the inside to the outside by the centrifugal force in the shearelement (13). The slots or teeth of the rotor or stator wall throughwhich the graft rubber dispersion is transported by centrifugal forceare arranged in the radially outer region of stator and rotor, where theradial speed of the rotor is particularly high. During or after passagethrough the slots or gaps between the teeth, owing to the very highspeed of the rotor relative to the stator, such strong shearing or shearforce is exerted on the dispersion particles that the latter collidewith one another and the dispersion coagulates. The shear velocities arefrom 5 to 30, preferably from 6 to 25, m per second. The averageresidence time of the graft rubber dispersion in the apparatus (10) isin general less than 12, preferably less than 10, particularlypreferably less than 8, seconds. The apparatus (10) contains in generalfrom 1 to 3 shear elements (13, 14, 15) connected in series, preferablyone shear element or two shear elements, it being possible for theirrotors and stators to have at least one row of teeth which is formed byaxial slots, or a plurality of concentrically arranged rows or rings ofslots or teeth. In addition to single-slot rotors, rotors which haveadditional blades in the axial and/or radial direction and rotors whichare provided with turbine blade-like displacer strips in addition to theshear elements are often advantageous, leading to better producttransport and a better suction effect on the dispersion.

The coagulated graft rubber dispersions subjected to shearing in one ormore stator/rotor combinations as shear elements are generally obtainedin the form of a free-flowing powder or in the form of a moist sludge,which leave the apparatus (10) via a radially arranged outlet orificeand can then be fed directly, ie. without being transported byextruders, to a kettle containing hot water, the use of superatmosphericpressure and of water temperatures above 100° C. being advantageous insome cases. The coagulated graft rubber can be worked up in a simplemanner by known methods for further use. In the novel coagulation byshearing, the dispersion used, the precipitation quality, thethrough-put and the desired coagulum form determine the details of themode of operation of the apparatus, ie. they determine, for example, theadvantageous number of stator/rotor combinations, the slot arrangementor tooth system of rotor and stator and any choice of particular rotortypes. This can be determined and optimized in a very limited number ofsimple experiments. The common purpose is in general to achieve completecoagulation of the polymer dispersion used with a high throughput andwith constant overall composition of the polymers and to obtaincoagulums which can be further processed very readily.

BRIEF DESCRIPTION OF DRAWINGS

The apparatus used according to the invention is illustrated by way ofexample in FIGS. 1-3.

FIG. 1 shows a longitudinal section through such an apparatus, with aperspective view of the rotor,

FIG. 2A shows a plan view of a stator from the direction of arrow A inFIG. 1,

FIG. 2B shows a side view of a partial cross-section of the stator ofFIG. 2A,

FIG. 2C shows a view of a rotor from the direction of arrow A in FIG. 1,

FIG. 2D shows a side view of a partial cross-section of the rotor ofFIG. 2C,

FIG. 3A shows a plan view of a further embodiment of a stator from thedirection of arrow A in FIG. 1,

FIG. 3B shows a side view of a partial cross-section of the stator ofFIG. 3A,

FIG. 3C shows a plan view of a further embodiment of a rotor from thedirection of arrow A in FIG. 1 and

FIG. 3D shows a side view of a partial cross-section of the rotor ofFIG. 3C.

A very suitable apparatus (10) for the novel process for the continuouscoagulation of graft rubber dispersions by shearing is shown inlongitudinal section in FIG. 1, the rotor being shown in perspectiveview. Such apparatuses are commercially available as dispersingapparatuses, ie. are used for a purpose exactly opposite to coagulationby shearing. Arranged axially in the interior of a housing (11) of theapparatus (10) are two or three shear elements (13, 14, 15), each ofwhich consists of a circular stator (13 a, 14 a, 15 a) as a stationarytool (also see FIGS. 2A and 3A) and of the rotor (13 b, 14 b, 15 b) as arotating tool rotating within the circular stator. The rotors (13 b, 14b, 15 b) (also see FIGS. 2C and 3C) are arranged on a rotatably mounteddrive shaft (12) generally connected to a motor. The stators and rotorsare essentially pot-like, in each case the ring-like wall of thepot-like stator (13 a, 14 a, 15 a) radially overlapping the wall of theassociated pot-like rotor (13 b, 14 b, 15 b) by a certain length. Thewalls are provided with axially oriented, radially continuous slots aspassages. The graft rubber dispersion to be coagulated is sucked intothe apparatus (10) via an inlet orifice (16) arranged in the axialextension of the drive shaft (12) and is fed axially to the first rotor(13 b). Therein, the dispersion is caused to rotate rapidly and istransported by the centrifugal force radially from the inside to theoutside, via the slots present in the stator (13 a) and rotor (13 b),into an annular gap defined between the rotor (13 b) and stator (13 a).Owing to the high speed of the circumference of the rotor (13 b)relative to the stator (13 a), a steep shear gradient forms in theturbulent flow in the annular gap. The particles of the graft rubberdispersion therefore collide at high speed and coagulate as a result ofthe high shear forces. The at least substantially coagulated dispersionemerges radially from the apparatus (10) via the outlet orifice (17),which is at the top in FIG. 1 but may also be at the bottom in anotherpreferred embodiment.

As stated, rotor and stator are slotted, the axially oriented, radiallycontinuous rotor slots advantageously being provided at or near thecircumference of the rotor. Rotor and stator may also have a pluralityof concentrically arranged slotted or toothed rings, toothed beingconsidered to be the same as slotted (cf. for example FIG. 2C). Thus,the first rotor (13 b) in the apparatus (10) in FIG. 1 has a row ofteeth which runs between two radially spaced rows of teeth of the stator(13 a). There are therefore two annular gaps in which the coagulation byshearing can take place. The stator (13 a) shown in FIG. 2A and 2B hasan inner toothed ring (21) and an outer toothed ring (22). The outerstator wall is denoted by (23). The stator shown in FIGS. 3A and 3B hasonly one toothed ring (21).

The rotor (13 b) shown in FIG. 2C and 2D has an inner toothed ring (25)and an outer toothed ring (26). The rotor shown in FIG. 3C and 3Dcomprises shear elements (27) and turbine blades (impellers) (28).

The coagulation apparatuses used for the novel process areadvantageously of multistage form and contain a plurality of, forexample 2 or 3, rotor/stator combinations connected in series as shearelements. However, depending on the physical properties of the graftrubber dispersion, it may also be advantageous to use only onerotor/stator combination or two rotor/stator combinations as shearelements, in order to achieve an optimum between precipitation quality,throughput and product properties. FIG. 1 shows an apparatus havingthree shear elements (13, 14, 15) connected in series. The rotors andstators of the shear elements may have slots or teeth of the same typeor of different types. Thus, FIG. 1 shows, in the apparatus (10), afirst shear element (13) which contains a stator (13 a) having tworadially spaced rows of teeth and a rotor (13 b) having only one row ofteeth. The second shear element (14) of the apparatus (10) has a stator(14 a) having three rows of teeth and a rotor (14 b) having two rows ofteeth which once again are concentrically nested one within the other.The rotor (14 b) furthermore has more teeth than the rotor (13 b) of thefirst shear element (13), which are therefore narrower and a smallerdistance apart. The third shear element (15) of the apparatus (10)likewise has a stator (15 a) having three rows of teeth and a rotorhaving two rows of teeth, the rotor (15 b) once again having more teeththan the rotor (14 b) of the second shear element (14).

In the shear elements used for the novel process, the flow of thedispersion is deflected repeatedly so that the dispersion particlescannot pass unprocessed through the apparatus.

Apparatuses similar to that in FIG. 1 and used in further, particularlypreferred embodiments are those which contain only one shear element ortwo shear elements (stator/rotor pairs), in order thus to optimize thethroughput and the precipitation quality. The space left by the omittedshear element or shear elements, for example the space (13) left by theomitted stator/rotor combination (13 a/ 13 b), is preferably filled byan exactly fitting displacer element so that no additional dead volumesare formed in the apparatus. A cylindrical displacer element replacingthe stator/rotor combination (13 a/ 13 b), for example in the space(13), exactly fills the space left by the stator/rotor combination,whose central bore corresponds to the internal diameter of the feedline. This apparatus innovatively modified compared with commerciallyavailable apparatuses has proven surprisingly advantageous not only forthe novel process but generally for apparatuses of this type for thepreparation or coagulation of polymer dispersions.

As stated, apparatuses which can be used for the novel process for thecontinuous coagulation of graft rubber dispersions, with the exceptionof the apparatus described above and modified with displacer elements,are known and obtainable as dispersing apparatuses. They are thereforeused for preparation and distribution of fine polymer particles in theaqueous dispersion, whereas they serve in the novel process fordestroying the aqueous polymer dispersion by producing larger coagulatedpolymer particles. The present invention therefore furthermore relatesto the use of these apparatuses for coagulating aqueous dispersions ofgraft rubbers by shearing, the use of the described novel apparatuseshaving at least one displacer element generally relating to thedispersing and coagulation of polymer dispersions.

According to the invention, aqueous dispersions of graft rubbers arecoagulated, the process having proven particularly useful forcoagulating aqueous dispersions of graft rubbers which have elastomericacrylate polymers as the grafting base (P1). In the dispersions to becoagulated according to the invention, the graft rubbers are present inparticular with solids contents of from 20 to 65, preferably from 30 to60, % by weight. The novel process can therefore be used directly forthe dispersions as obtained by graft polymerization in aqueousdispersion.

Graft rubbers whose aqueous dispersions are coagulated according to theinvention are understood as meaning graft polymers in which monomers, inparticular styrene, acrylonitrile and/or methyl methacrylate, forminghard thermoplastics are grafted, as graft shell (P2), onto particlecores comprising soft rubber (P1), this being effected by polymerizationor copolymerization of the monomers for the graft shell (P2) in thepresence of the rubber particles (P1). In this procedure, some of themonomers or resulting polymers form a linkage with the surface of therubber particles, which can be increased by specific known processmeasures (increase of grafting yield). By repeating the graftingmeasure, it is also possible to prepare graft rubbers having more thanone graft shell (P2).

Suitable soft rubbers (grafting base P1) for the preparation of thegraft rubbers are elastomeric polymers and/or copolymers having glasstransition temperatures of less than −10° C. and preferably less than−30° C. Elastomeric 1,3-diene homo- and copolymers, such as butadiene,isoprene or chloroprene homo- and copolymers, preferably butadienerubber, and elastomeric acrylic ester homo- and/or copolymers having thestated low glass transition temperatures are particularly suitable.Elastomeric acrylic ester polymers are preferred for the graft rubberscoagulated according to the invention, for example homo- and copolymersof C₄-C₈-alkyl acrylates, in particular of n-butyl acrylate and/or2-ethylhexyl acrylate. Examples of preferred comonomers of the alkylacrylates are crosslinking monomers having at least two nonconjugatedC═C double bonds, such as diallyl maleate, diallyl phthalate,diacrylates and dimethacrylates of diols, such as 1,4-butanediol or1,6-hexanediol, etc., preferably allyl methacrylate ordihydrodicyclopentadienyl acrylate, which are used in particular in anamount of from 0.5 to 10% by weight of the total amount of monomers inthe elastomer preparation, and furthermore polar monomers, such asacrylic acid, methacrylic acid, maleic anhydride, acrylamide,methacrylamide, N-methylolacrylamide or N-methylolmethacrylamide and thealkyl ethers thereof. The amount of elastomers (P1) in the graft rubber(P) is in general from 30 to 85% by weight, and the novel process canalso advantageously be used for processing, preferably, graft rubbers(P) which contain more than 50, for example from about 55 to about 85, %by weight, based on the total solids content, of elastomer (P1).

Particularly suitable monomers for polymerizing on the graft shell (P2)are monomers and mixtures thereof which form hard polymers or copolymershaving glass transition temperatures of more than +50° C. The type ofthe monomer or monomers for this purpose depends substantially on thetype of thermoplastics which, after blending with the graft rubber, willform the polymer matrix and with which the graft shell should have acertain compatibility or affinity in order to achieve ,a fine two-phasedistribution of the graft rubbers in the matrix. Particularly suitableand conventional monomers for the graft shell are vinyl- andalkenylaromatic monomers of 8 to 12 carbon atoms, such as styrene,α-methylstyrene, and styrenes and α-methylstyrenes which carry one ormore alkyl, in particular methyl, groups as substituents on the benzenenucleus. They may be the sole monomers for the preparation of the graftshell (P2) or may be used as a mixture with other monomers, such asmethyl methacrylate, methacrylonitrile and, preferably, acrylonitrile,the graft shell containing from 0 to 45, preferably from 10 to 40, % byweight, based on the graft shell, of methacrylonitrile and/oracrylonitrile monomer units. Mixtures of styrene with from 10 to 40% byweight, based on the total amount of monomers, of acrylonitrile arepreferred. Preferred further monomers for the preparation of the graftshell are also methacrylic esters and acrylic esters, among which methylmethacrylate is preferred and may also be used as the sole orpredominant monomer for the preparation of the graft shell. However,maleic anhydride, maleimide, N-phenylmaleimide, acrylic acid andmethacrylic acid are also among the suitable comonomers for thepreparation of the graft shell (P2).

Particularly suitable for the novel coagulation process are graft rubberdispersions which were prepared by grafting the elastomers with themonomers for the graft shell, at least in the last process stage of thegraft polymerization, in aqueous emulsion and then particularly in thepresence of anionic emulsifiers. Conventional anionic emulsifiers are,for example, alkali metal salts of alkanesulfonic or alkylarylsulfonicacids, alkylsulfates, fatty alcohol sulfonates, salts of fatty acids of10 to 30 carbon atoms or resin soaps. Emulsions prepared using sodiumsalts of fatty acids of 10 to 18 carbon atoms (anionic soaps) oralkanesulfonates as anionic emulsifiers are very suitable. The anionicemulsifiers are used in amounts of from 0.5 to 5, in particular from 1to 2, % by weight, based on the amount of monomers. An excess ofemulsifier is avoided where subsequent coagulation of the graft rubberemulsions by shearing is intended.

The Examples and Figures which follow are intended to illustrate thenovel process in more detail but not to limit it. Parts and percentagesare by weight, unless stated otherwise.

EXAMPLE 1

a) Preparation of the graft rubber dispersion with polybutadiene as thegrafting base

60 parts of butadiene were polymerized to a monomer conversion of 98% ina solution of 0.6 part of tert-dodecyl mercaptan, 0.7 part of sodiumC₁₄-alkanesulfonate as emulsifier, 0.2 part of potassium peroxodisulfateand 0.2 part of sodium disulfate in 80 parts of water at 65° C. In theresulting latex, the polybutadiene had a mean particle size of 100 nmand was therefore agglomerated by adding 25 parts of a 10% strengthemulsion of a copolymer of 96% of ethyl acrylate and 4% ofmethacrylamide, a mean particle size of 350 nm resulting. The glasstransition temperature of the polybutadiene was −85° C.

40 parts of water, 0.4 part of sodium C₁₄-alkanesulfonate and 0.2 partof potassium peroxodisulfate were added to the product.

40 parts of a mixture of 70% of styrene and 30% of acrylonitrile wereadded gradually in the course of 4 hours and the batch was kept at 75°C. while stirring. The monomer conversion was virtually quantitative.The glass transition temperature of a copolymer of 70% of styrene and30% of acrylonitrile was about +105° C.

b) Coagulation of the dispersion prepared according to a)

The coagulation of the graft rubber dispersion was carried out by thenovel process, the apparatus used being a Dispax 3/6/6 dispersingapparatus from Janke & Kunkel, which contained two rotor/statorcombinations (both coarse) with a stator external diameter of about 60mm and a rotor external diameter of 55 mm as shear elements. Otherwise,the apparatus was similar to the apparatus (10) shown in FIG. 1. Thedispersion was fed axially to the apparatus at 77° C. In the first shearelement (13), the stator 2 and the rotor 2 had toothed rings (cf. FIG.2) and the space 14 remained empty, and, in the second shear element(15), the stator and the rotor each had a toothed ring, the rotor alsocontaining turbine blade-like displacer strips (cf. FIG. 3). The speedof the rotors was 7500 revolutions per minute and the throughput of theapparatus was 200 kg/hour. A coagulum which had a dry appearance, aresidual moisture content of 57% and a mean particle size of 1 mm inconjunction with a broad particle size distribution (0.05-5 mm) emergedfrom the apparatus. The coagulum was introduced in free fall into astirred kettle containing hot water. The product could then be readilydewatered by decanting, centrifuging and drying. The resulting productcould then be used directly as an impact modifier by mixing intostyrene/acrylonitrile copolymers in an extruder.

EXAMPLE 2

a) Preparation of the graft rubber dispersion with polybutadien as thegrafting base

The preparation was carried out as described in Example 1a).

b) Coagulation of the dispersion prepared according to a)

The coagulation of the graft rubber dispersion was carried out by thenovel process, the apparatus used being a Dispax 3/6/6 dispersingapparatus from Janke & Kunkel. The apparatus is similar to the apparatus(10) shown in FIG. 1. The first shear element (13) was replaced by afitted cylindrical displacer element having a central bore, whichelement completely filled the space (13) without dead space, and theinternal diameter of the central bore corresponded to the internaldiameter of the feed line. The shear element (14) was a stator/rotorcombination (14 a/ 14 b) having coarse teeth. The shear element (15) hada stator/rotor combination (15 a/ 15 b) according to FIG. 3. The speedwas 7500 revolutions per minute, the temperature 75° C. and thethroughput 300 kg/hour. Crumbly product was obtained and was worked upas stated in Example 1. The moisture content was 55.3%, the particlesize on average was 1 mm and the particle size distribution was broad(0.04-4 mm).

EXAMPLE 3

a) Preparation of the graft rubber dispersion with elastomericpolyacrylate as the grafting base

160 parts of a mixture of 98% of butyl acrylate and 2% ofdicyclopentadienyl acrylate were heated to 60° C., while stirring, in1500 parts of water with the addition of 5 parts of the sodium salt of aC₁₂-C₁₈-paraffinsulfonic acid, 3 parts of potassium peroxodisulfate, 3parts of sodium bicarbonate and 1.5 parts of sodium diphosphate. 15minutes after the beginning of the polymerization reaction, a further840 parts of the monomer mixture were added in the course of 3 hours.After the end of the monomer addition, the emulsion was kept at 60° C.for a further hour. The glass transition temperature of the elastomerwas −42° C.

1150 parts of water and 2.7 parts of potassium peroxodisulfate wereadded to 2100 parts of the emulsion, and the mixture was heated to 65°C. while stirring. After this temperature had been reached, 560 parts ofa mixture of 75 % of styrene and 25 % of acrylonitrile were metered inover 3 hours. After the end of the addition, the batch was kept at 65°C. for a further 2 hours. The glass transition temperature of acopolymer of 75% of styrene and 25% of acrylonitrile was +111° C.

b) Coagulation of the dispersion prepared according to a)

The coagulation was carried out by the novel process, the apparatus usedbeing a laboratory apparatus which was similar to the shearing apparatus(10) shown in FIG. 1 and which had, as the process section, two shearelements (14, 15) each having a slotted rotor (14 b, 15 b) and eachhaving a slotted stator (14 a, 15 a) firmly connected to the housing.The shear element (13) was replaced by an exactly fitted cylindricaldisplacer element having a central bore. The external diameter of thestators (14 a, 15 a) was about 60 mm and that of the rotors (14 b, 15 b)about 53 mm.

Stator (14 a) and rotor (14 b) are shown schematically in FIG. 2, andstator (15 a) and rotor (15 b) in FIG. 3. The gap width between rotortooth system and stator tooth system was about 0.75 mm. The graft rubberdispersion was fed axially to the apparatus at 60° C. and transported inthe shear elements radially from the inside to the outside. The rotorspeed was 8000 revolutions per minute and the throughput 440 kg/hour. Aproduct in the form of a moist slurry was discharged radially at theoutlet orifice (17). It was converted into larger agglomerates byheating with steam under superatmospheric pressure to above the glasstransition temperature T_(g) of the graft shell, and said agglomeratescould be readily dewatered by decanting, centrifuging and drying. Theresulting product was very suitable as an impact modifier for thepreparation of ASA polymers.

List of reference numerals 10 Apparatus 11 Housing 12 Drive shaft 13First shear element 13a First stator 13b First rotor 14 Second shearelement 14a Second stator 14b Second rotor 15 Third shear element 15aThird stator 15b Third rotor 16 Inlet orifice 17 Outlet orifice 21 innertoothed ring of the stator 22 outer toothed ring of the stator 23 outerstator wall 25 inner toothed ring of the rotor 26 outer toothed ring ofthe rotor 27 shear elements 28 Turbine blade

We claim:
 1. A process for the continuous coagulation of aqueousdispersions of graft rubbers (P) which a) contain, as soft grafting base(P1), elastomeric polymers and/or copolymers of 1,3-dienes or acrylicesters having a glass transition temperature T_(g) of less than −10° C.in an amount of from 30 to 85% by weight of the amount of graft rubber(P) and b) contain a hard one-shell or multishell graft shell (P2)comprising monomer units which form a thermoplastic polymer or copolymerhaving a glass transition temperature of more than +50° C. and whosecontent of acrylonitrile or methacrylonitrile monomer units is from 0 to45% by weight, and c) at least in the last stage of the graftpolymerization, were prepared by emulsion polymerization of the monomersfor the graft shell (P2) in the presence of the grafting base (P1),which process comprises transporting the graft rubber through anapparatus (10) having at least one shear element (13, 14, 15) with astationary circular slotted stator (13 a, 14 a, 15 a) and a slottedrotor (13 b, 14 b, 15 b) which rotates within the stator and is mountedon a rotatably mounted drive shaft (12), so that the graft rubberdispersion introduced into the apparatus (10) is passed radially fromthe inside to the outside as a result of the rotation of the rotor (13b, 14 b, 15 b) in the shear element (13, 14, 15) and, during or afterpassage through the slots of the rotor (13 b, 14 b, 15 b) and stator (13a, 14 a, 15 a), is subjected to shearing having a shear velocity in theshear element (13, 14, 15) of from 4 to 30 m/second which is so strongthat coagulation of the graft rubber dispersion takes place.
 2. Aprocess as claimed in claim 1, wherein the apparatus (10) contains twoor three shear elements (13, 14, 15), each having a stator/rotorcombination.
 3. A process as claimed in claim 1, wherein the graftrubber dispersion is introduced axially into the apparatus (10) throughan inlet orifice (16) and fed to the first shear element (13 a) and,after having passed the shear element or elements (13, 14, 15), theresulting coagulated product is discharged from the apparatus (10)through a radially arranged outlet orifice (17).
 4. A process as claimedin claim 1, wherein the apparatus (10) contains a plurality of shearelements (13, 14, 15) and each rotor (13 b, 14 b, 15 b) and stator (13a, 14 a, 15 a) has, for shearing, from one to three concentricallyarranged rows of teeth formed by axial slots.
 5. A process as claimed inclaim 1, wherein the coagulation is effected in the shearing apparatuswith only one shear element or two shear elements (13, 14, 15), and thespace left by at least one omitted shear element is filled by adisplacer element to avoid dead volumes.
 6. A process as claimed inclaim 1, wherein at least one rotor (13 b, 14 b, 15 b) which has bladesarranged in the axial or radial direction is used.
 7. A process asclaimed in claim 1, wherein at least one of the rotors (13 b, 14 b, 15b) has turbine blade-like displacer strips.